Many mechanical systems include rotating shafts for coupling or transmitting rotational energy from one location to another. Examples include but are not limited to the shafts used in cars, boats, planes, power generation turbines and compressors. In many cases it is necessary or desirable to measure the motion of the shaft in operation. Specific measurements of interest include but are not limited to rotational velocity, rotational acceleration, rotational vibration (also called torsional vibration), lateral vibration, twist and torque. These measurements may be necessary to control the motion of the shaft during operation, to monitor the shaft or machine for motions that indicate problems or potential failures, or to evaluate and optimize the performance of the machine during design or manufacture.
Contact potential difference (CPD) measurements have been used as one form of sensor technology. CPD refers to the fact that different metals have different electon energies and work functions, and that when two metals are placed in electrical contact, electrons flow from the metal with the lower work function to the metal with the higher work function. If two different metals are connected electrically and then brought in close proximity, then an electric filed is formed between them as a result of their CPD.
“Vibrating CPD sensor” refers to the vibration of one metal relative to the other in a parallel plate capacitor system. That is, the vibrating CPD sensor operates by electrically connecting two metals and vibrating one metal of known work function (the probe) over the second metal of unknown work function. The vibration induces changes in capacitance over time. A time varying signal is generated that is a function of the capacitance and the voltage that results from the CPD between the two metals. A variable electrical potential is applied between the two metals and adjusted until the time-varying current is eliminated. The resulting applied voltage is the CPD between the two metals. The work function of the unknown metal can then be calculated as the work function of the probe plus or minus the applied potential. Non-vibrating CPD sensors (nvCPD) are also known. Such probes are not vibrated, but rather the probe and the testing surface are moved relative to each other. This translation makes high speed scanning possible.
Several options exist to monitor the motion of a rotating shaft. The most common method of measuring rotary shaft motion is to use an encoder. An encoder is a device that attaches to a shaft and converts rotary motion into electrical pulses. Encoders can provide precise information on shaft motion, but they are relatively expensive and require substantial effort to mount on a shaft. Lower-cost or temporary encoders for measuring rotary motion can be built by mounting some sort of target to the shaft and then detecting the presence or location of the target using a separate sensor. For example, magnetic targets can be mounted to a shaft and detected using Hall Effect sensors, or a patterned tape can be mounted on the shaft and motion detected using an optical sensor. These techniques require the effort and expense of modifying the shaft so that its motion can be detected, and are limited to detecting rotary motion. It is also possible to measure shaft motion without modifying the shaft by mounting optical or eddy current sensors above a gear to detect the presence or motion of gear teeth. This technique is limited in that it can only be used at the locations of gears in the system.
Several techniques also exist for measuring lateral vibrations of a rotating shaft. These include the use of laser interferometers or eddy current sensors to detect the distance between the sensor and the surface of the shaft. Vibrations can also be detected using accelerometers mounted to various locations on the machine. These techniques are limited to measuring lateral vibration and, in the case of accelerometers, measuring machine vibrations and not the vibration of the shaft itself. In general, it can be expensive, time-consuming, and difficult to precisely measure both the lateral and rotary motions of a rotating shaft.